Generally, a molten salt includes at least one salt, and is used in a molten state. In recent days, salts that are in the molten state (that is, liquid state) even under a normal temperature have been proposed, and they are called a normal temperature molten salt, an ionic liquid, and so on.
Molten salts are used, for example, as an electrolyte used in industrial electrolysis or metal refining, and also used in manufacture of metal plutonium in atomic energy systems. Particularly, since a large amount of electric current flow is necessary in industrial electrolysis or metal refining, the molten salt used as an electrolyte is required to have high ion conductivity.
Molten salts are also used as an electrolyte of thermal batteries. Thermal batteries generally include a plurality of unit cells each of which having a negative electrode, a positive electrode, and an electrolyte disposed therebetween. For the electrolyte of such batteries, a salt that melts at high temperature (i.e., molten salt) is used. At normal temperature, electrolytes of molten salts have no ion conductivity and therefore thermal batteries are inactive. However, when the electrolyte is heated to high temperature, the electrolyte achieves a molten state, becoming an excellent ion-conductor. Thus, thermal batteries become active under high temperature and can supply electricity to the outside.
Thermal batteries are a type of storage battery, and normally the battery reaction does not advance unless the electrolyte melts. Thus, even after storage period of 5 to 10 years or more, thermal batteries can bring out the same level of battery performance as fresh batteries. Also, in thermal batteries, electrochemical reactions advance under high temperature. Therefore, compared with batteries using aqueous electrolytes or organic electrolytes, the electrochemical reaction advances far more rapidly. Thermal batteries thus have excellent large current discharge performance. Furthermore, in thermal batteries, electric power will be readily available in a short period of time, i.e., within a second, when an activation signal is sent to the battery upon use, though the period varies depending on the heating means (heating element). Therefore, thermal batteries are suitably used for a power source of various ordnance devices such as guidance systems, an emergency power source, and so on.
Meanwhile, in these days, devices with thermal batteries mounted therein are achieving increasingly high performance, and therefore power consumption of these devices is also increasing. Thus, thermal batteries are required to achieve even higher output. For achieving an output of about the same level as in conventional thermal batteries, by using the techniques for achieving high output, the batteries can be made small and lightweight compared with those batteries made by conventional techniques. Therefore, if high output can be achieved, by using the techniques for achieving high output, demands for small and lightweight batteries can be met.
Various examinations have been conducted on electrolytes so far for achieving high output thermal batteries. For example, Patent Document 1 has proposed that a LiF—LiCl—LiBr molten salt is used as an electrolyte. Patent Document 2 has proposed that a LiCl—LiBr—KBr molten salt is used as an electrolyte. Non-patent Document 1 has proposed that a molten salt containing iodine salt such as LiF—LiCl—LiI is used as an electrolyte.
Patent Document 1: Japanese Laid-Open Patent Publication No. Hei 2-61962
Patent Document 2: Japanese Laid-Open Patent Publication No. Hei 10-172581
Non-Patent Document 1: P. Masset, Journal of Electrochemical Society, 152(2), A 405-A 410 (2005)
To achieve high output, ion conductivity (conductivity) of the molten salt has to be high. Meanwhile, in terms of practical use, the melting point of the molten salt has to be also considered. Even with high ion conductivity, when the molten salt has a higher melting point than the melting point of the currently used molten salt, a special material has to be used for a heating element disposed in the battery. Furthermore, the operation temperature range of the thermal battery is also limited. That is, a molten salt with a high melting point has significant demerits.
For example, among the molten salts that have been proposed so far, the molten salt with the highest ion conductivity is probably the LiF—LiCl—LiBr molten salt disclosed in Patent Document 1 above. However, the melting point of this molten salt is 443° C., which is 90° C. or more higher than the melting point of a widely used LiCl—KCl molten salt (melting point 350° C.). Thus, when the LiF—LiCl—LiBr molten salt is used as an electrolyte, the internal temperature setting of the thermal battery has to be made far higher than the conventional setting of about 500° C. When for example FeS2, which starts thermal decomposition at about 600° C., is used as a positive electrode material and the internal temperature setting of the thermal battery is set far more higher than 500° C., there may be a possibility of positive electrode material deterioration.
Furthermore, in many cases, thermal batteries are required to be dischargeable even in an environment of −50° C. In such a low temperature environment, the temperature is about 70° C. lower than the normal temperature (about 20° C.). Therefore, when the operating temperature of thermal batteries under a normal temperature environment is set to 500° C., i.e., a general operating temperature of thermal batteries, in an environment of low temperature such as −50° C., the internal temperature of the thermal battery will probably be about 430° C. Since the melting point of the LiF—LiCl—LiBr molten salt disclosed in Patent Document 1 is about 440° C., when the internal temperature of the thermal battery decreases to about 430° C., the temperature of the molten salt also reaches the range of the freezing point. As a result, the thermal battery containing the molten salt disclosed in Patent Document 1 can only provide a very small discharge capacity, or becomes unable to discharge.
As described above, when the molten salt disclosed in Patent Document 1 is used, and the operating temperature of the thermal battery is set high, it gives severe effects on the positive electrode and so on. Additionally, when a thermal battery containing the molten salt disclosed in Patent Document 1 is used in a low temperature environment, discharge performance and so on will be affected.
The melting point of the LiCl—LiBr—KBr molten salt disclosed in Patent Document 2 is about the same as the melting point of the LiCl—KCl molten salt. However, the ion conductivity of the LiCl—LiBr—KBr molten salt is low compared with the ion conductivity of the LiCl—KCl molten salt. Therefore, even if the molten salt disclosed in Patent Document 2 is used, output performance of thermal batteries cannot be improved.
Non-patent Document 1 discloses a molten salt containing iodine salt such as LiF—LiCl—LiI. The molten salt containing iodine salt has a melting point of about 400° C. and a relatively high ion conductivity depending on the iodine salt content. The molten salt disclosed in Non-patent Document 1 contains a relatively high concentration of iodine salt. However, as described later, iodine has a far higher reactivity with moisture or oxygen compared with other salts. Therefore, when the molten salt disclosed in Non-patent Document 1 is used as an electrolyte of a thermal battery, further improvement in terms of practical use is necessary.